The present invention relates generally to thermal energy management systems within electron beam generating devices and, more particularly, to an assembly for cooling and relieving stress from an x-ray tube window.
There is a continuous effort to increase x-ray imaging system scanning capabilities. This is especially true in computed tomography (CT) imaging systems. Customers desire the ability to increase the peak power to reduce X-ray doses. The increase in peak power also allows physicians to get improved CT images of vascular applications with high speed CT imaging systems. Although the increase in imaging speed generates improved imaging capability, it causes new constraints and requirements for the functionality of the CT imaging systems.
CT imaging systems include a gantry rotating at various speeds in order to generate a 360° image. The gantry includes an x-ray tube, which composes a large portion of the rotating gantry mass. The CT tube generates x-rays across a vacuum gap between a cathode and an anode. For generating the x-rays, a large voltage potential is applied across the vacuum gap allowing electrons to be emitted, in the form of an electron beam, from the cathode to the anode target. During the releasing of the electrons, a filament contained within the cathode is heated to incandescence by passing an electric current therein. The electrons are accelerated by the high voltage potential and impinge on the target, whereby they are abruptly slowed down to emit x-rays. The high voltage potential generates a large amount of heat within the x-ray tube, especially within the anode.
Typically, a small portion of energy within the electron beam is converted into x-rays; the remaining electron beam energy is converted into thermal energy within the anode. The thermal energy radiates to other components within a vacuum vessel of the x-ray tube and is removed from the vacuum vessel via a cooling fluid circulating over an exterior surface of the vacuum vessel. Additionally, electrons within the electron beam are back-scattered from the anode and impinge on other components within the vacuum vessel, causing additional heating of the x-ray tube. As a result, the x-ray tube components are subject to high thermal stresses decreasing component life and reliability of the x-ray tube.
The vacuum vessel is typically enclosed in a casing filled with circulating, cooling fluid, such as dielectric oil. The casing supports and protects the x-ray tube and provides for attachment to a computed tomography (CT) system gantry or other structure. Also, the casing is lined with lead to provide stray radiation shielding. The cooling fluid often performs two duties: cooling the vacuum vessel, and providing high voltage insulation between the anode and cathode connections in the bi-polar configuration.
High temperatures at an interface between the vacuum vessel and a transmissive window in the casing cause the cooling fluid to boil, which may degrade the performance of the cooling fluid. Bubbles may form within the fluid and cause high voltage arcing across the fluid, thus degrading the insulating ability of the fluid. Further, the bubbles may lead to image artifacts, resulting in low quality images.
Prior art cooling methods have primarily relied on quickly dissipating thermal energy by using a circulating, coolant fluid within structures contained in the vacuum vessel. The coolant fluid is often a special fluid for use within the vacuum vessel, as opposed to the cooling fluid that circulates about the external surface of the vacuum vessel. Other methods have been proposed to electromagnetically deflect back-scattered electrons so that they do not impinge on the x-ray window.
These approaches, however, are limited with regard to energy storage and dissipation. Due to inherent poor efficiency of x-ray generation and desire for increased x-ray flux, heat load is increased that must be dissipated. As power of x-ray tubes continues to increase, heat transfer rate to the coolant can exceed heat flux absorbing capabilities of the traditional cooling system design.
A thermal energy storage device or electron collector, coupled to an x-ray window, has been used to collect back scattered electrons between the cathode and the anode. In using this device, the collector and window need to be properly cooled to prevent high temperature and thermal stresses, which can damage the window and joints between the window and collector.
High temperature on the window and collector can induce boiling of coolant. Bubbles from boiling coolant obscure the window and thereby compromise image quality. Further boiling of the coolant results in chemical breakdown of the coolant and sludge formation on the window, which also results in poor image quality.
Previously, a heat exchange chamber has been coupled to the electron collector, including a cooling channel, which allows coolant to flow in the channel across each of four walls of the electron collector. Although, the heat exchange chamber aids in cooling the electron collector, it is difficult to effectively manufacture due to its complexity and large number of seams, which each need to be properly sealed. Also, the heat exchange chamber has limited effectiveness in cooling of and preventing deposits from forming on the x-ray tube window. Also, portions of the window have been known to crack due to cyclic thermal loadings.
It would therefore be desirable to provide an apparatus and method of cooling an x-ray tube or x-ray tube window, that allows for increased scanning speed and power, that is relatively easy to manufacture, and that minimizes blurring and artifacts in a reconstructed image.